Wells used directly for oil or gas production deliver the oil or gas through tubing positioned within a cased borehole. Production can be assisted by pumping gas into the annulus surrounding the production tubing and passing it through gas lift mandrels into the production tubing. Wells used indirectly for oil or gas production may inject water into the reservoir below the oil layer or pump excess gas back into the reservoir above the oil layer.
Gas lifting is the primary artificial lift method used in offshore oil wells. However, the reservoir draw-downs possible with gas lift are not as high as those that can be achieved with the assistance of pumps. The proper application of pumps can lower the abandonment pressure of wells, increasing reserves captured per well, and reducing the number of wells required to economically deplete an asset. Unfortunately, high-volume oilfield submersible pumping systems are plagued by various issues that reduce their applicability, particularly in high-cost offshore environments and horizontal directionally drilled wells.
Electric submersible pumps (ESPs) or progressive cavity pumps (PCP) are the primary high-volume pumping options available to industry today. ESPs have reliability issues caused by induction motor, seal section, shaft, and power cable failures. The seal section is particularly troublesome, as it is designed to provide a physical barrier between the motor internals and the wellbore fluids. When the seal fails, wellbore fluids can reach the thrust bearings and/or motor, resulting in system failure. ESPs must be specially designed to handle produced gases, which further limits their use. ESPs are commonly installed as part of the tubing string, which means they require a costly pulling rig for installation and replacement.
Modern wells, for safety reasons and regulatory requirements, are typically equipped with subsurface safety valves (SSSV) for offshore applications and other environmentally sensitive areas. Subsurface safety valves are generally designed to be self-closing valves and may be placed down the well, both in the production tubing and the annulus. Subsurface safety valves may be surface controlled and may have springs that fail-safe in the closed position in case of an emergency. Under normal production conditions, they may be kept open by hydraulic fluid pressure.
Motors for supplying the energy to ESP and PCP applications are typically installed in the wellbore, below the position of the SSSV, hence their size is limited, maintenance is difficult and costly, and reliability can be an issue.
As an alternative, specifically for subsea applications, a pump motor installed on top of the wellhead would enable the use of larger motors and be easier to maintain, while providing enhanced artificial lift forces. However, employing a motor on top of the wellhead requires a rotating shaft extending from the wellhead, down the wellbore, to the pump, prohibiting the use of conventionally available subsurface safety valves, since such valves would be unable to close in an emergency.
Therefore, what is needed is a subsurface safety valve that is designed to function with an artificial lift system having a pump motor positioned on top of the wellhead and a rotating shaft extending therefrom, which is effective in sealing the wellbore quickly in an emergency.